Currently, three-dimensional (3-D), solid-state memories based on polysilicon diodes and antifuses are beginning to be commercialized and promise to be less expensive than the current low-cost leader in solid-state memory, two bit-per-cell NAND flash memory. 3-D memories increase chip capacity by a large factor over conventional memories. In this way, cost per bit can be significantly reduced. However, vertically stacked memories produced to date have limited application, because they can not be rewritten. Also, only one bit per cell can be stored, because the antifuses are either blown or not-blown.
In one simple approach, a rewritable, variable-resistance memory element would take the place of the antifuses, and would be compatible with a polysilicon diode. Therefore, it is expected that such a memory element would be unipolar with the same direction of current flow for both writing and erasing the memory element, and that it would be able to withstand the high temperatures used for polysilicon diode fabrication. Moreover, it is expected that the current density during operations of writing and erasing the memory cell should not exceed the current-carrying capacity of polysilicon diodes.
Many skilled in the art feel that phase-change memory (PCM) has the best chance to compete with flash memory in the future. Although a PCM is a unipolar, variable-resistance device, it requires a high current during reset, and is not stable at high temperature. In conventional two-dimensional (2-D) PCM, these problems are avoided, because single crystal diodes are grown directly up from the silicon substrate at high temperature before any temperature sensitive phase-change (PC) material is deposited in fabrication. Unfortunately, this approach is not feasible for a vertically stacked memory device, such as a 3-D PCM, because of the temperature sensitivity of the PC material to any subsequently deposited polysilicon in which current-steering elements, e.g. diodes, are formed.